Another feature of note seen on the iOS 8 feature list: WiFi Calling.
WiFi calling routes phone calls and SMS messages over WiFi if available, allowing for calls to get through even when cellular service is unavailable.
It only works with supported carriers - but it can be a great way to keep connected when out of site of a tower.
T-Mobile has had WiFi calling on a few select phones since 2007, and Sprint has begun to offer it on a few phones as well.
Verizon and AT&T on the other hand have not enabled WiFi calling yet, probably because their cellular coverage areas are much more pervasive so that the need for WiFi calling support is a lot less urgent.
The catch with WiFi calling -- if you are using a limited data connection (like a MiFi) the data used to make a voice call will end up chewing up your data plan. Mobile users will need to be careful to make sure that their cheap T-Mobile phone doesn't chew through all their Verizon MiFi data!